Unseptunium
Unseptunium, Usu, is the temporary name for element 171. NUCLEAR At least one set of theoretical values for half-lives and decay modes of Usu have been constructed for neutron count up to N = 333(1). It predicts isotopes ranging from Usu 439 to Usu 503 in three bands: Usu 439 to Usu 447, Usu 464 to Usu 485, and Usu 500 to Usu 503. Examination of pp 15 & 18 of Ref. 1 indicates that Usu 464 through Usu 485 are part of the expected pattern of alpha-decaying nuclei centered on the N = 308 shell closure. Isotopes between Usu 469 and Usu 480 are predicted to have sub-millisecond half-lives and to decay by alpha emission. The other isotopes in this band have sub-microsecond half-lives and most decay by alpha emission, which is consistent with neutron shell closure at Usu 479. Usu 439 to Usu 447 are very neutron-poor, and not located in the vicinity of any suggested shell closure; they appear to be artifacts of the sort common near the edges of models. Usu 500 to Usu 503 might indicate a shell closure near N = 330, but they are also located in a region whose patterns of half-lives and decay modes indicates that the model may have reached the limits of its capability. What Ref. 1 can’t do is describe heavy isotopes of Usu. It is possible to use a first-order, liquid-drop approach to guess at what lies there. At least two computations of the neutron dripline’s location up to Z = 175 exist(2),(3), and since they give similar results, the maximum possible size of a Usu nucleus can be set slightly above the values computed, allowing only a small margin for error. This gives Usu 612 as the heaviest possible Usu isotope. Similarly, a realistic lower bound can be set by requiring that the amount of energy needed to stabilize a nucleus be no more than twice what is needed to stabilize Usp 471. Within this range, the liquid-drop model can be used to indicate the amount of structural correction energy needed to allow a drop of nuclear matter to survive for the 10^-14 sec needed for electromagnetic interactions (such as binding an electron) to become important. Structural correction required for Usu 612 itself is a little over 1 MeV, which means all Usu drops will fission quickly without structural stabilization. In general, it is not possible to describe structural correction energy. What can be predicted are neutron and proton shell closures, for which correction energy is expected to be particularly large. Neutron shell closures have been predicted at N = 406(3),(4), 370(3), 318(5), and 308(1). The isotope Usu 577 requires 1.5 MeV of structural correction, which means isotopes in the Usu 567 to Usu 582 band are likely. (See “Formation” for additional significance of these nuclei.) Usu 541 requires 1.5 MeV of structural correction, which means isotopes in the band Usu 531 to Usu 546 are also likely. All isotopes in both bands should beta-decay with half-lives under a second. On the other hand, Usu 489 requires 4.5 MeV of correction energy, which means it is likely to stabilize some nuclei in its vicinity. Ref. 1 does not show a pattern of nuclides which indicate a shell closure at N = 318. Usu 479 requires 6 MeV of correction energy, which is realistic for a strong neutron shell closure, such as the one predicted at N = 308, so the liquid-drop picture isn’t unrealistic. ATOMIC Several predictions for the ground state electron structure of Usu agree that it will have p-block character, with two 9s, two 9p1/2, and three 8p3/2 electrons available for bonding. Electrons in Usu can probably be described in terms of time-independent orbitals, but there is a some chance that the conventional time-independent orbital concept does not apply to atoms with this high a value of Z. Calculation of electron properties require that nuclear charge be distributed over the nucleus' actual volume. In addition, there is some chance that differing nuclear shapes may produce different electron configurations in different isotopes. (Different isotopes would be different elements in the chemical sense.) Except in the laboratory, Usu is expected to exist only in environments too hot for ordinary chemistry to occur. FORMATION Ions of this element may form when material from roughly 1 km depth is ejected from a disintegrating neutron star during a merger. There is a possibility that beta decay from dripline nuclides stabilized by the N = 406 closure, enhanced by the Z = 164 proton shell closure, will allow some isotopes in the vicinity of Usu 566 to Usu 577 to form in quantity during such a merger. It improbable that nuclides between Usu 531 and Usu 546, or lighter, can form in this way. Fusion or multinucleon transfer reactions in the polar jets emanating from a neutron star or black hole might produce lighter isotopes, including those in the Usu 464 to Usu 485 band. Quantities produced by this method are very small. REFERENCES 1. "Decay Modes and a Limit of Existence of Nuclei"; H. Koura; 4th Int. Conf. on the Chemistry and Physics of Transactinide Elements; Sept. 2011. 2. "Neutron and Proton Drip Lines Using the Modified Bethe-Weizsacker Mass Formula; D.N. Basu et al; Int.J.Mod.Phys.; arXiv:nucl-th/0306061; url: https://arxiv.org/abs/nucl-th/0306061 3. “Single Particle Levels of Spherical Nuclei in the Superheavy and Extremely Superheavy Mass Region”; H. Koura and S. Chiba; Journal of the Physical Society of Japan; DOI 10.7566/JPSJ.82.014201; Jan. 2013. 4. "Magic Numbers of Ultraheavy Nuclei"; V. Yu Denisov; Physics of Atomic Nuclei, v. 68, no. 7, pp 1133-1137; 2005. 5. “The Highest Limiting Z in the Extended Periodic Table”; Y.K. Gambhir, A. Bhagwat, and M. Gupta; Journal of Physics G: Nuclear and Particle Physics. 42 (12): 125105. DOI:10.1088/0954 3899/42/12/ 125105. (12-11-19)